An image reading apparatus used as, e.g., the scanner unit of a digital copying machine reads an original image by irradiating an original to be read with light from a light source and receiving the reflected light by a reading sensor. As the reading sensor, a line sensor which reads the image of one line in the main scanning direction is used. The reading operation for each line is repeatedly executed while relatively moving the reading region and original in the sub-scanning direction perpendicular to the main scanning direction, thereby reading a two-dimensional image.
As the light source, a fluorescent lamp long in the main scanning direction is used. The light source is relatively moved with respect to the original together with the reading region of the reading sensor.
To read an original set on the original table, it is only necessary to irradiate a region corresponding to the original existing region with the light source. There is proposed an image reading apparatus which detects the dimension in the sub-scanning direction of an original set on the original table and changes the range in which the light source unit is relatively moved in the sub-scanning direction in accordance with the dimension in this direction (e.g., Japanese Unexamined Patent Publication No. 2003-37714).
In this technique, the sub-scanning irradiation range is adjusted in accordance with the original dimension in the sub-scanning direction. However, the region in the main scanning direction is always irradiated with light throughout the length of the light source. For this reason, if an original whose dimension in the main scanning direction is shorter than the light source is set on the original table and read while keeping the platen cover open, light that irradiates the original non-existing region dazzles the eyes. In addition, even an unnecessary region is irradiated with light, and the energy is wasted.
This application is based upon and claims priority under 35 U.S.C. 119 from the Japanese Patent Application No. 2004-196675 filed in Jul. 2, 2004, at least entire content of which are incorporated herein by reference.